How to restore without updating OS?

This morning I tried installing MyWi through Rock, and rebooted. Now my phone begins to boot showing the pineapple as usual, but after about 20 seconds shuts off. Tried many times with same result.

So I guess I need to restore to factory settings, then restore my back-up? My worry is that the downloaded restore file will be 3.1.3, which I will not be able to jailbreak. Will this be the case, and how can I work around it?

On the 3G and 2G you can just download the proper firmware from any multitude of websites and hold SHIFT ( or Command if on a mac ) and press the restore button and it will let you browse for the firmware you downloaded.

I have basically the same problem. My Iphone is acting weird and I need to restore but I don't want to update to 3.1.3. Could someone PM and give me a website that has instructions on how to do the custom firmware thing?